The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Production of oil and gas from subterranean formations presents a myriad of challenges. One such challenge is the lack of permeability in certain formations. Often oil or gas bearing formations, that may contain large quantities of oil or gas, do not produce at a desirable production rate due to low permeability; the low permeability causing a poor flow rate of the sought-after hydrocarbons. To increase the flow rate, a stimulation treatment can be performed. Once such stimulation treatment is hydraulic fracturing. Hydraulic fracturing is a process whereby a subterranean hydrocarbon reservoir is stimulated to increase the permeability of the formation, increasing the flow of hydrocarbons from the reservoir. A fracturing fluid is pumped at very high pressure, e.g., in excess of 10,000 psi, to crack the formation thereby creating larger passageways for hydrocarbon flow.
While the high pressure introduced may produce cracks in a formation, the removal of the pressure back to normal borehole pressures, often cause the closing of the cracks much in the manner that a crack wedged open in a piece of wood may close when the wedge used to produce the crack is removed. Such closing of the reservoir cracks produced by the hydraulic fracturing operating is very undesirable.
To avoid the closing of reservoir cracks when the hydraulic pressure is lowered, the fracturing fluid may have proppants added thereto, such as sand or other solids that fill the cracks in the formation, so that, at the conclusion of the fracturing treatment, when the high pressure is released, the cracks remain propped open, thereby permitting the increased hydrocarbon flow possible through the produced cracks to continue into the wellbore.
In order to pump the fracturing fluid into the well, large oilfield operations generally employ any variety of positive displacement or other fluid delivering pumps.
A positive displacement pump may be a fairly large piece of equipment with associated engine, transmission, crankshaft and other parts, operating at between 200 Hp and about 4,000 Hp. A large plunger is driven by the crankshaft toward and away from a chamber in the pump to dramatically affect a high or low pressure thereat. This makes a positive displacement pump a good choice for high pressure applications. Hydraulic fracturing of underground rock, for example, often occurs at pressures between 10,000 to 20,000 PSI or more.
When employing oilfield pumps, regular pump monitoring and maintenance may be sought to help ensure uptime and increase efficiency of operations. A pump, as with any form of industrial equipment, is susceptible to natural wear that could affect uptime or efficiency. This may be of considerable significance in the case of pumps for large-scale oilfield operations as they are often employed at the production site and operated at a near round the clock basis and may operate under considerably harsh protocols. For instance, in the case of hydraulic fracturing applications, a positive displacement pump may be employed at the production site and intended to operate for six to twelve hours per day for more than a week generating extremely high pressures. Thus, wear on pump components during such operation may present in a variety of forms.
Abrasive wear occurs when the particles within the fluid impact on the exposed surfaces of the machinery and impart some of their kinetic energy into the exposed surface. If sufficiently high, the kinetic energy of the impacting particles creates significant tensile residual stress in the exposed surface, below the area of impact. Repeated impacts cause the accumulation of tensile stress in the bulk material that can leave the exposed surface brittle and lead to cracking, crack linkage and gross material loss.
In particular, internal valve seals of the pump are prone to failure, especially where abrasive oilfield material is directed through the pump during a fracturing application. These internal valve seals may be of a conformable material in order to allow proper sealing. However, the conformable nature of the seal may leave it susceptible to deterioration by abrasive oilfield materials that are pumped through the valves. Additionally, other components of the pump may be susceptible to wear by abrasives that are pumped through the pump. Such deterioration of pump components may considerably compromise control over the output of the pump and ultimately even render the pump ineffective.
Efforts have been made to actually prevent pump damage by pumped abrasives. These efforts include introducing abrasives, such as proppants, at locations subsequent to the pressure producing valves and other particularly susceptible oilfield pump components. For example, as detailed in U.S. Pat. No. 3,560,053 to Ortloff, a pressurized abrasive slurry may be introduced to an oilfield fluid after the fluid has been directed from an oilfield pump. In this manner, the oilfield pump may be spared exposure to the potentially damaging abrasive slurry.
Unfortunately, the method described above, is achieved by the addition of a significant amount of equipment at the oilfield. Often this equipment may require its own monitoring and maintenance due to exposure to the abrasive slurry. For example, mixing and blending equipment along with pressurization equipment, including susceptible valving, may be required apart from the primary oilfield pumps described above. Thus, while the original pumps may be spared exposure to abrasives, another set of sophisticated equipment remains exposed.
Because the fracturing fluid is pumped at extremely high pressure, the proppants included in the fracturing fluid can be coated in order to increase their durability and use under high-pressure conditions and to minimize proppant flow back from propped hydraulic fractured oil and gas wells. The coating of proppants is well known in the state of the art. In U.S. Pat. No. 5,597,784 to Sinclair et al, a method is disclosed for coating the proppant in a resin. Proppants are typically coated in a factory or at a location remote to the well site and transported to the well site after coating has been applied.
Transporting the coated proppant to the well site means that the choices for materials with which the proppant can be coated are limited to those types of coatings that will not sustain damaged in the shipping process. Also, when the proppant is received at the well site and pumped through the high-pressure pumps, the proppant is at risk to become damaged within the processing equipment.
In addition to coatings, stimulation fluid is often augmented with other additives to aid in the stimulation or propping operations. Such additives include lubricants, viscosity breakers, friction reducing agents, cross-link delaying agents, fiber, explosive chemicals, bonding agents, and adhesives. It is desirable that these additives are mixed with the proppant prior to introduction into the high-pressure flow of a hydraulic stimulation treatment.
From the foregoing it will be apparent that there remains a need for a system of pumping abrasive slurry that does not impact the wear and tear of an oilfield pump or pump components.
From the foregoing it will be apparent that there remains a need for a proppant coating mechanism that offers improved process control over the proppant coating process. Furthermore, from the foregoing it will be apparent that it would be desirable to provide a mechanism for introducing proppant and related additives as a mixture without requiring pumping of such a mixture through the high-pressure pumps used to produce the hydraulic pressure used to stimulate hydrocarbon reservoirs.